106 MORPHOLOGY AND EVOLUTION
thickened crests which do not meet across the nose. The size of the cavities is dependent on the law which states that the volume of the eyes is inversely proportional to the bulk of the body. The optic foramen is round, and separated from the triangular anterior lacerate foramen. The inferior orbital fissure is wide and long. The os planum of the ethmoid bone is sometimes divided into two parts by a vertical suture. The lachrymal kamulus is rudimentary, and the bony bed for the lachrymal sac may be deep. The naso-lachrymal duct runs down to the nose through a well-marked canal.
The anterior narial aperture is ovoid, and is bounded below by the preruaxillse, laterally by the nasal processes of the maxillae, and above by the flat, quadrate bone formed from the fused nasal bones. The premaxillsB do not extend upwards on the sides of the narial aperture to meet the nasal bones. No anterior nasal spine is present. On looking into the aperture one sees the turbinate bones and bony septum nasi very plainly.
The infra-orbital foramina consist of one prominent foramen placed on the maxillae below the junction of the inner and middle thirds of the inferior margin of the orbit, and of two foramina on the malar bone. Through the former the infra-orbital branch of the maxillary division of the trigeminal nerve emerges.
No infra-orbital suture appears on the face. No supra-orbital foramen exists.
The most anterior part of the maxillae, lying between the infra-orbital foramen and the anterior narial aperture, is expanded to form the long socket of the large